The art of printing images with micro-fluid technology is relatively well known. Conventionally, a permanent or semi-permanent ejection head has access to a local or remote supply of fluid. The fluid ejects from an ejection zone of the head to a print media in a pattern corresponding to pixels of images being printed. Over time, the heads and fluid drops have become increasingly smaller.
In the course of developing heads with fluid drop sizes smaller than 5 Pico liters, a “tree vein” or “wood grain” print defect has been observed. It consists of dark-toned bands meandering from outboard edges of a printing swath toward a center. The bands are typically present for most of the swath length except for a short portion near the beginning of fluid jetting. The bands have been also observed across any swath width so long as the fluid jetting nozzles are spaced relatively closely together. While reduction of the print gap from the ejection zone of the head to the print media tends to minimize or eliminate the defects, there is a lower practical limit to decreasing the gap. If it becomes too short, inadvertent contact with the media by the head will smear the yet-to-dry fluid.
In the print gap, air velocity varies approximately linearly between the scan speed at the ejection zone (e.g., nozzle plate) of the head and zero at the print medium. Simulation by the inventors has shown that a curtain of fluid drops from a closely spaced array of nozzles in an ejection zone is capable of strongly influencing the print gap airflow. The wakes of the drops effectively constitute a moving barrier that pushes out air as the head scans. The result is a flow field similar to a river flowing around a row of bridge pilings, in which the fluid velocity downstream meanders from side to side in irregularly shifting patterns. It is believed main drops from the head tend to travel to the print media with little deviation due to their large mass. The smaller satellite drops, on the other hand, are believed to slow down and become influenced in direction by the local airflow. The observed wood grain effect is consistent with this hypothesis, i.e., satellite drops are channeled together into concentrated bands by the print gap airflow as modified by the wakes of the main drops.
Simulations further show that the flow field around the head develops in both time and space. This effect occurs in the print gap also: the velocity profile changes with time and varies across the width of the ejection zone even when no fluid ejectors are jetting. The time-dependence of the no-jetting flow field likely contributes to the wood grain print defect by forcing and enhancing local velocity oscillations around the ejectors.
Accordingly, a need exists to minimize or eliminate printing defects, especially when utilizing small volume drops. The need further extends to modifying airflow in the print gap and to do so consistently across as much of the gap as possible. Additional benefits and alternatives are also sought when devising solutions.